Downing Street described the evidence as "limited but persuasive", but did not say whether it believed the regime or the rebels had been responsible for their use.

The White House said the US intelligence community had assessed with "varying degrees of confidence" that chemical weapons had been used by the regime on a "small scale".

But while President Obama has previously said the use of chemical warfare was a "red line" issue for the US, administration officials emphasised that the findings would not be an "automatic trigger" for military intervention.

Instead, both London and Washington called on Mr Assad to allow United Nations inspectors to conduct a full investigation to establish what had happened.

There was a clear determination in both capitals that - after the experience of Iraq - they should not be drawn into precipitate action on the basis of intelligence which could turn out to be faulty.

Downing Street described the findings - including evidence of the use of the deadly nerve agent, sarin - as "extremely concerning".

"We have limited but persuasive information from various sources showing chemical weapon use in Syria, including sarin," a No 10 spokesman said. "Use of chemical weapons is a war crime. We have briefed our allies, partners and the UN on this information and we are working actively to get more and better information."

It is understood that samples smuggled out of Syria were analysed by the Defence Science and Technology Laboratory at Porton Down, Wiltshire.